R1:efccd75ae371

Correct various minor diff copy behaviors

Summary:
Ref T12822. Fixes a few things:

- Firefox selection of weird ranges with an inline between the start and end of the range now works correctly.
- "Show More Context" rows now render, highlight, and select properly.
- Prepares for nodes to have copy-text which is different from display-text.
- Don't do anything too fancy in 1-up/unified mode. We don't copy line numbers after the `content: attr(data-n)` change, but that's as far as we go, because…